Evaluation of the Impact of Two Thiadiazole Derivatives on the Dissolution Behavior of Mild Steel in Acidic Environments

In light of the variety of industrial uses and economic relevance of mild steel, corrosion resistance is a serious topic. Utilization of inhibitors serves as one of the most essential methods for corrosion control. Two thiadiazole compounds, namely, 2-amino-5-(4-bromobenzyl)-1,3,4-thiadiazole (a1) and 2-amino-5-(3-nitrophenyl)-1,3,4-thiadiazole (a2), were synthesized. The structure of the prepared compounds was verified by Fourier transform infrared spectroscopy (FTIR) and proton and carbon-13 nuclear magnetic resonance spectroscopy (1H NMR and 13C NMR). In a 0.50 M H2SO4 solution, the effectiveness of two synthetic thiadiazole derivatives as mild steel corrosion inhibitors were investigated. In this evaluation, various electrochemical methodologies have been utilized, such as potentiodynamic polarization, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS). The results confirm the efficiency of the inhibition increases by raising concentrations of a1 and a2. The inhibitory behavior was explained by the notion that the adsorption of thiadiazole molecules, a1 and a2, on the surface of mild steel causes a blockage of charge and mass transfer, protecting the mild steel from offensive ions. Furthermore, the synthesized molecules a1 and a2 were analyzed using density functional theory (DFT).


Introduction
Mild steel (M-steel) is an economical and frequently utilized construction material in a broad range of industries due to its cost-effectiveness, low carbon content, and superior mechanical performance. Mild steel is susceptible to severe corrosion in acidic media [1][2][3][4][5][6][7][8]. Acidic solutions, particularly H 2 SO 4 acid solutions, are commonly utilized in both chemical and industrial processes [9,10]. Inhibitors are among the most commonly utilized practical strategies for maintaining metals in corrosive environments [11][12][13]. Several organic molecule inhibitors possessing heteroatoms such as nitrogen, sulfur, oxygen, or, various links such as amine, alcoholic, and carbonyl groups work through adsorption on the surface of metal [14,15]. Corrosion cannot harm the metal because the inhibitor coating adsorbs the entire corroded metal surface. The surface charge and origin of metallic materials, the chemical composition of corrosion inhibitors, and the type of aggressive solution all have an impact on the corrosion process [16].
When organic inhibitors contain specific halogen ions, their effectiveness can be increased. Furthermore, it is known that in acidic conditions, halogen ions can partially prevent corrosion. The order of corrosion inhibition effectiveness from most effective to least effective is iodide, bromide, and chloride ions. Fluoride does not possess any features that function in corrosion inhibition. The metal adsorbs halogen ions, whose charge move the surface in a negative direction, causing synergism of halogen ions which increases adsorption to the cationic organic inhibitor. It takes a lot of effort, creativity, and laboratory analysis/synthesis to identify potential chemicals that can be employed as corrosion inhibitors [17][18][19].
Many researchers have utilized numerous organic heterocyclic compounds as anticorrosion agents. In particular, heterocyclic molecules with sulfur, oxygen, and nitrogen atoms, for example, thiazole, oxazole, and azole derivatives, have shown significant beneficial efficiency against metallic corrosion in various conditions [20].
In comparison to compounds containing only nitrogen or sulfur atoms, heterocyclebased inhibitors, which contain both elements in their structure, provide exceptional corrosion-preventive effects. In this respect, a number of nitrogen-and sulfur-containing azole compounds, such as thiazole and thiadiazole derivatives, have been shown to be effective inhibitors of corrosion on several metallic materials in many different corrosive conditions. According to the bulk of the literature studying corrosion, thiadiazole-based compounds are more important than thiazole-based compounds. This trend in attention is based on the idea that adding more heteroatoms (nitrogen atoms) to such heterocyclic compounds can increase their adsorption to the metal surface and hence increase the efficiency of their inhibition [21][22][23].
Several applications of 1,3,4-thiadiazoles in industries such as pharmaceuticals and agrochemicals have been thoroughly investigated. They act as inhibitor enhancers and decrease metal degradation created by their surroundings. Several 1,3,4-thiadiazole derivatives have been shown to be efficient anti-corrosion agents in various conditions. This is likely due to the higher orientation of heteroatoms with conjugated multiple bonds, which facilitates the adsorption of such compounds onto the metal surface; after that, a protective layer is formed that isolates the substrate from components of the solution [24,25].
It became clear that 1,3,4-thiadiazole derivatives' ability to prevent metal dissolution is affected by the shape and size of the attached substituents as well as their chemical characteristics [26,27].
The structure of 2-amino-5-(4-bromobenzyl)-1,3,4-thiadiazole (1) was confirmed on the basis of its spectral data through mass spectrometry which showed well defined fragments (m/e, % of relative abundance): 271.9 ([M + 2] , 98), 269 (M , 100) indicating the molecular formula C 9 H 8 BrN 3 S (Figure 1a). The 1 H NMR spectrum (DMSO-d 6 ) of the ABT compound exhibited all the expected peaks corresponding to the proposed structure. The primary amine -NH 2 signal appeared at 7.038 ppm. The two protons of CH 2 gave a signal in 4.107 ppm. The two symmetrical protons in CH at positions 2 and 6 in the phenyl ring gave rise to doublet of doublets (dd) in 7.225 ppm. The other two symmetrical protons in CH at positions 3 and 5 in the phenyl ring gave rise to doublet of doublets (dd) in 7.508 ppm (Figure 1b). The 13 C NMR spectrum (DMSO-d 6 ) of compound 1 exclusively confirmed the proposed structure. Two aromatic carbons at positions 2 and 5 in the thiadiazole ring showed two signals at 169.0 ppm and 157.3 ppm, respectively. The carbon of the methylene Molecules 2023, 28, 3872 3 of 17 group between the phenyl ring and the thiadiazole ring showed a signal at 35.2 ppm. Six carbons at positions 1, (2, 6), (3,5), and 4 in the phenyl ring showed four signals at 137.9, 131.4, 132.0, and 120.5, ppm respectively ( Figure 1c).  The structure of 2-amino-5-(4-bromobenzyl)-1,3,4-thiadiazole (1) was confirmed on the basis of its spectral data through mass spectrometry which showed well defined fragments (m/e, % of relative abundance): 271.9 ([M + 2] , 98), 269 (M , 100) indicating the molecular formula C9H8BrN3S (Figure 1a). The 1 H NMR spectrum (DMSO-d6) of the ABT compound exhibited all the expected peaks corresponding to the proposed structure. The primary amine -NH2 signal appeared at 7.038 ppm. The two protons of CH2 gave a signal in 4.107 ppm. The two symmetrical protons in CH at positions 2 and 6 in the phenyl ring gave rise to doublet of doublets (dd) in 7.225 ppm. The other two symmetrical protons in CH at positions 3 and 5 in the phenyl ring gave rise to doublet of doublets (dd) in 7.508 ppm ( Figure 1b). The 13 C NMR spectrum (DMSO-d6) of compound 1 exclusively confirmed the proposed structure. Two aromatic carbons at positions 2 and 5 in the thiadiazole ring showed two signals at 169.0 ppm and 157.3 ppm, respectively. The carbon of the methylene group between the phenyl ring and the thiadiazole ring showed a signal at 35.2 ppm. Six carbons at positions 1, (2, 6), (3,5), and 4 in the phenyl ring showed four signals at 137.9, 131.4, 132.0, and 120.5, ppm respectively ( Figure 1c).  The structure of 2-amino-5-(4-bromobenzyl)-1,3,4-thiadiazole (1) was confirmed on the basis of its spectral data through mass spectrometry which showed well   Figure 3b. It was determined, based on Figure 3b, that the effect of a1 is greater than that of a2. The Fe samples' OCP versus time curves were almost straight, signifying the achievement of a steady-state potential [28][29][30].

Potentiodynamic Polarization
Potentiodynamic polarization curves for the M-steel corrosion both in the blank and inhibited solution containing 0.005 M concentrations of a1 and a2 compounds in 0.50 M H 2 SO 4 medium are depicted in Figure 4.
The addition of the examined inhibitors, a1 and a2, into the solution had little impact on the general form of the potentiodynamic curves, showing that a1 and a2 merely block the reaction sites (active sites) of the M-steel surface without variation in the cathodic and anodic reaction mechanisms.
Adsorption represents the first step toward corrosion prevention in neutral solutions. The damping effect is created by the inhibitor molecules adsorbing onto the active corrosion locations on the M-steel surface. The chemical composition of the inhibitor, as well as the type and charge of the degraded surface, influence the adsorption mechanism [31]. This is because the surface of the M-steel to be inhibited is usually oxide-free, enabling the inhibitor's immediate proximity to delay the cathodic and/or anodic electrochemical processes. Although the adsorbed inhibitor does not completely cover the M-steel's surface, it does occupy electrochemically active sites and reduce the intensity of either the cathodic or anodic reactions, or both. The rate of corroding will be reduced in relation to the extent to which the electrochemically active areas are covered by the adsorbing inhibitor. From Table 2, it was detected that the addition of a1 to the sulfuric acid had a larger effect on the M-steel corrosion rate when compared to the a2 inhibitor.    Figure 3b. It was determined, based on Figure 3b, that the effect of a1 is greater than that of a2. The Fe samples' OCP versus time curves were almost straight, signifying the achievement of a steady-state potential [28][29][30].

Potentiodynamic Polarization
Potentiodynamic polarization curves for the M-steel corrosion both in the blank and inhibited solution containing 0.005 M concentrations of a1 and a2 compounds in 0.50 M H2SO4 medium are depicted in Figure 4. The addition of the examined inhibitors, a1 and a2, into the solution had little impact on the general form of the potentiodynamic curves, showing that a1 and a2 merely block the reaction sites (active sites) of the M-steel surface without variation in the cathodic and anodic reaction mechanisms.
Adsorption represents the first step toward corrosion prevention in neutral solutions. The damping effect is created by the inhibitor molecules adsorbing onto the active corrosion locations on the M-steel surface. The chemical composition of the inhibitor, as well as the type and charge of the degraded surface, influence the adsorption mechanism [31]. This is because the surface of the M-steel to be inhibited is usually oxide-free, enabling the inhibitor's immediate proximity to delay the cathodic and/or anodic electrochemical processes. Although the adsorbed inhibitor does not completely cover the M-steel's surface, it does occupy electrochemically active sites and reduce the intensity of either the cathodic or anodic reactions, or both. The rate of corroding will be reduced in relation to the extent to which the electrochemically active areas are covered by the adsorbing inhibitor. From Table 2, it was detected that the addition of a1 to the sulfuric acid had a larger effect on the M-steel corrosion rate when compared to the a2 inhibitor.   Figure 5 depicts the effect of increasing the concentration of a1 on potentiodynamic polarization plots for M-steel corrosion in H 2 SO 4 . It was discovered that the cathodic (hydrogen evolution reaction) and anodic (metal dissolution) mechanisms were altered as a result of adding the inhibitor to the aggressive media [31]. Several electrochemical parameters were computed from the extrapolation of Tafel branches, including cathodic (β c ) and anodic (β a ) Tafel slopes, corrosion current density (i cor ), and potential corrosion (E cor ) and are presented in Table 3. The inhibitory performance (IP%) and surface coverage (θ) were calculated using the following equation. IP% = 1 − i sur f i f ree × 100 = θ × 100, where, i free and i surf are the corrosion current density (i corr ) without and with a1 and a2 compounds, respectively. Table 3 shows that the inhibitory capability rises as the concentration increases, reaching around 97.1% in the presence of 0.005 M of a1. Such behavior could be understood by an enhanced adsorption of a1 molecules onto the Msteel/H 2 SO 4 solution interface [32,33], which is encouraged by increasing the surface coverage (cf. Table 3) Furthermore, there is no significant change in the E cor following the application of the investigated chemical. This indicates that a1 compounds function as mixed form inhibitors [34,35]. Inhibition efficiency percentages of our constructed thiadiazole derivatives are also shown in Table 4, along with the percentage of inhibition efficiency for various organic compounds that have been chosen and used as effective corrosion inhibitors in various conditions. The PP measurements with low doses of thiadiazole derivatives were used to derive the inhibitory efficacy values listed in this table [36][37][38][39][40][41][42][43]. Our two newly created thiadiazole derivatives (a1) are more potent inhibitors than other chosen chemical derivatives, as shown in Table 4.   Inhibition efficiency percentages of our constructed thiadiazole derivatives are also shown in Table 4, along with the percentage of inhibition efficiency for various organic compounds that have been chosen and used as effective corrosion inhibitors in various conditions. The PP measurements with low doses of thiadiazole derivatives were used to derive the inhibitory efficacy values listed in this table [36][37][38][39][40][41][42][43]. Our two newly created thiadiazole derivatives (a1) are more potent inhibitors than other chosen chemical derivatives, as shown in Table 4. 1-(2-ethylamino-1,3,4-thiadiazol-5-yl)-3-phenyl-3-oxopropan (ETO) 500 ppm 98.4 [37] N-cyanoacetohydrazide (CAH) 500 ppm 48.24 [38] N-acryloylN0-cyanoacetohydrazide (ACAH) 500 ppm 91.1 [38] poly(N-acryloyl-N0-cyanoacetohydrazide) (PACAH) 500 ppm 96.57 [38] 1-Amino-2-mercapto-5-(4-(pyrrol-1-yl)phenyl)-

Adsorption Isotherm
Polarization measurements were utilized to evaluate surface coverage (θ) values for adsorption of various a1 concentrations on the surface of M-steel. Many adsorption isotherms were graphically examined to identify the most appropriate adsorption isotherm for a1 adsorption on the surface of M-steel [44], Figure 6. The Langmuir adsorption isotherm was identified to be the closest match, and it is derived from the subsequent equation. [44,45]: C/θ = (1/K) + C, where C is the a1 concentration and K is the equilibrium constant of adsorption. Figure 6 reveals the graphing of C/θ vs. C, which produces straight lines with almost unit slopes for a1 with an intercept of 1/K. The relationship between the standard free energy of adsorption (∆G • ads ) and equilibrium constant of adsorption (K) is described by the next equation [29]: K = (1/55.5) exp −∆G o ads /TR , where T refers to the absolute temperature, H 2 O molar concentration, and R refers to the gas constant. In 0.5 M H 2 SO 4 , the free energy of adsorption for adsorbed a1 on the iron surface is 31 kJ·mol −1 , while the equilibrium constant is 5000. The fact that ∆G o ads is negative indicates that a1 adsorbs spontaneously on the iron surface [45][46][47]. In this investigation, all of the processes implicated in the system's electrical response were matched with an equivalent Randle CPE circuit model (see inset Figure 7a). Where the resistance of the solution (Rs) describes ohmic resistance, the charge transfer resistance (Rct) represents the inhibitor's resistance to metal surface oxidation and is inversely proportional to the rate of corrosion. A constant phase element (CPE) replaces a pure double layer capacitor (C dl ) to justify the semicircle form of the Nyquist plot [48]. The larger the diameter of the Nyquist plot, the larger the Rct value, and consequently, the better the inhibitory efficacy of a particular inhibitor. lines with almost unit slopes for a1 with an intercept of 1/K. The relationship between the standard free energy of adsorption (ΔG°ads) and equilibrium constant of adsorption (K) is described by the next equation [29]: K 1 55.5 ⁄ exp ∆G TR ⁄ , where T refers to the absolute temperature, H2O molar concentration, and R refers to the gas constant. In 0.5 M H2SO4, the free energy of adsorption for adsorbed a1 on the iron surface is 31 kJ·mol −1 , while the equilibrium constant is 5000. The fact that ∆G o ads is negative indicates that a1 adsorbs spontaneously on the iron surface [45][46][47].  Figure 7a,b shows EIS plots for M-steel corrosion in both the blank and inhibited solutions containing 0.002 M quantities of a1 and a2 chemicals in 0.50 M H2SO4 medium. In this investigation, all of the processes implicated in the system's electrical response were matched with an equivalent Randle CPE circuit model (see inset Figure 7a) . Where the resistance of the solution (Rs) describes ohmic resistance, the charge transfer resistance (Rct) represents the inhibitor's resistance to metal surface oxidation and is inversely proportional to the rate of corrosion. A constant phase element (CPE) replaces a pure double layer capacitor (Cdl) to justify the semicircle form of the Nyquist plot [48]. The larger the diameter of the Nyquist plot, the larger the Rct value, and consequently, the better the inhibitory efficacy of a particular inhibitor. According to Figure 7, the results of these experiments show that the film resistances for M-steel have maximum values in the presence of a1. The impact of rising a1 concentration on EIS plots for M-steel corrosion in a 0.50 M H2SO4 environment is shown in Figure 8. In general, raising the inhibitor concentration causes the observed impedance values to rise, indicating that the electrode surface becomes more passive . This could be related to the strengthening of the inhibitive layer on the mild steel surface [49]. The Bode According to Figure 7, the results of these experiments show that the film resistances for M-steel have maximum values in the presence of a1. The impact of rising a1 concentration on EIS plots for M-steel corrosion in a 0.50 M H 2 SO 4 environment is shown in Figure 8. In general, raising the inhibitor concentration causes the observed impedance values to rise, indicating that the electrode surface becomes more passive. This could be related to the strengthening of the inhibitive layer on the mild steel surface [49]. The Bode plot figures show, at the intermediate frequency, one maximum phases. According to ohmic law, the greater the resistance value (Rct), the smaller the electrical current (I) flow, and hence the fewer the number of electrons that are transmitted over the surface of M-steel. Consequently, the metal dissolving process (iron oxidation) appears to be impeded [50]. The fact that the Nyquist plots retained semicircle forms throughout the studies demonstrated that corrosion protection occurs via a charge transfer mechanism [51,52]. The results of the impedance measurements were in good agreement with those of the potentiodynamic polarization experiments.

Theoretical Studies
Quantum chemical calculation is a molecular structure-based approach to problem solving that involves the computation of molecular parameters that have a substantial correlation with predictable response functions such as inhibition effectiveness. The thiadiazole derivatives under study had their geometry optimized at the DFT level using the functional B3LYP and basis set 6-31 G (d,p) using BIOVA Materials Studio 7.0 (Accelrys Inc., San Diego, CA, USA). According to reports, HOMO orbitals provide electrons, whereas LUMO orbitals accept electrons [53,54]. Figures 9 and 10 display the optimized molecular structures of the a1 and a2 compounds with the HOMO and LUMO electronic densities that were obtained from theoretical calculations. The HOMO orbitals surrounding the benzyl ring on the a2 molecule, according to the results from Figures 9 and 10. For a1, the HOMO orbitals are dispersed throughout the entire molecule. On the other hand, the LUMO orbitals are located on the thiadiazole ring group for a2 and over the entire molecule for a1. This demonstrates the molecules' electron-donating center. The results of DFT demonstrate that a1 and a2 have various electron donor and acceptor sites. This also explains the difference in inhibition effectiveness between a1 and a2. The theoretical computations produced findings that are compatible with the practical results.

Theoretical Studies
Quantum chemical calculation is a molecular structure-based approach to problem solving that involves the computation of molecular parameters that have a substantial correlation with predictable response functions such as inhibition effectiveness. The thiadiazole derivatives under study had their geometry optimized at the DFT level using the functional B3LYP and basis set 6-31 G (d,p) using BIOVA Materials Studio 7.0 (Accelrys Inc., San Diego, CA, USA). According to reports, HOMO orbitals provide electrons, whereas LUMO orbitals accept electrons [53,54]. Figures 9 and 10 display the optimized molecular structures of the a1 and a2 compounds with the HOMO and LUMO electronic densities that were obtained from theoretical calculations. The HOMO orbitals surrounding the benzyl ring on the a2 molecule, according to the results from Figures 9 and 10. For a1, the HOMO orbitals are dispersed throughout the entire molecule. On the other hand, the LUMO orbitals are located on the thiadiazole ring group for a2 and over the entire molecule for a1. This demonstrates the molecules' electron-donating center. The results of DFT demonstrate that a1 and a2 have various electron donor and acceptor sites. This also explains the difference in inhibition effectiveness between a1 and a2. The theoretical computations produced findings that are compatible with the practical results.
the LUMO orbitals are located on the thiadiazole ring group for a2 and over the entire molecule for a1. This demonstrates the molecules' electron-donating center. The results of DFT demonstrate that a1 and a2 have various electron donor and acceptor sites. This also explains the difference in inhibition effectiveness between a1 and a2. The theoretical computations produced findings that are compatible with the practical results.

Materials
The working electrode utilized in the present work is mild steel (M-steel) emb in epoxy holders with an exposed area of 0.12 cm 2 . The chemicals used in our invest include Thiosemicarbazide, 4-bromophenyl acetic acid, 3-nitro benzoic acid, phos oxychloride, phosphorus oxychloride, potassium hydroxide, and ethanol. All chem agents were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Equipment and Instrumentation
All melting points are given uncorrected in °C. They were computed on a MEL II melting point instrument. The 1 H NMR spectra of the compounds produced study were measured using a Bruker (300 MHz) spectrometer, while their IR spe KBr were measured using a Pye Unicam SP 1200 spectrometer.

Materials
The working electrode utilized in the present work is mild steel (M-steel) embedded in epoxy holders with an exposed area of 0.12 cm 2 . The chemicals used in our investigation include Thiosemicarbazide, 4-bromophenyl acetic acid, 3-nitro benzoic acid, phosphorus oxychloride, phosphorus oxychloride, potassium hydroxide, and ethanol. All chemical reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Equipment and Instrumentation
All melting points are given uncorrected in • C. They were computed on a MEL-TEMP II melting point instrument. The 1 H NMR spectra of the compounds produced in this study were measured using a Bruker (300 MHz) spectrometer, while their IR spectra in KBr were measured using a Pye Unicam SP 1200 spectrometer.

Electrochemical Measurements
The electrochemical corrosion studies were carried out at room temperature using Versa STAT 4 and the Versa Studio Electrochemical software suite. The working, reference, and auxiliary electrodes in the three-electrode system in the jacketed glass cell were made of mild steel, calomel, and Pt, respectively. The working electrode was attached using epoxy resin, with 0.12 cm 2 of its surface exposed to the tested hostile media. Before each run, the exposed region was polished to a mirror-like surface using grades up to 2500 grit. After 15 min of immersion in the test solution, the electrochemical tests were performed. At an open-circuit potential (OCP), potentiostatic circumstances were used for the EIS measurements, which covered the frequency range from 100 kHz to 0.1 Hz at an amplitude of 10 mV. The potentiodynamic polarization responses were investigated in a 0.5 M H 2 SO 4 solution at 25 • C and a scan rate of 5 mV/s in without any corrosion inhibitors or in the presence of different quantities of corrosion inhibitors (0.0005, 0.001, 0.002, 0.003, 0.004, and 0.005 M).

Conclusions
Two thiadiazole compounds, a1 and a2, were synthesized and their chemical structures identified. Both a1 and a2 demonstrated the ability to create a protective coating on the M-steel surface against acidic media. 2-amino-5-(4-bromobenzyl)-1,3,4-thiadiazole (a1) demonstrates higher inhibition efficiency than 2-amino-5-(3-nitrophenyl)-1,3,4-thiadiazole for M-steel, and the inhibition efficiency increased as a function of inhibitor concentration. Surprisingly, a negative ∆G 0 ads implies that adsorption occurs spontaneously. The synthesized thiadiazole compounds adsorb on the surface of M-steel through a combination of chemical and physical processes that correlate to the Langmuir adsorption isotherm. The corrosion potential values did not change significantly, indicating that compounds a1 and a2 serve as mixed-type inhibitors. Theoretical studies based on DFT agree with our research observations demonstrating the efficacy of a1 and a2 as corrosion additives.